Noble metal doped chalcogenide glasses, such as silver selenide, are presently of great interest for use in non-volatile memory devices. Non-volatile memory devices employing thin films of doped chalcogenide glasses offer several advantages over conventional devices, especially with regard to non-volatile memory speed, thermal characteristics, durability and reliability.
Thin film doped deposition is an important aspect of fabricating memory devices containing metal doped chalcogenide glasses. Because these films play an important role in the electrical performance of such memory devices, it is desirable that these deposited layers be defect free. This is especially true when such memory devices are used in ultra large scale integration (ULSI) devices, whose sub-micron structure greatly increases the need for thin film defect control. For example, thin film defects may be 10 to 100 times larger than typical ULSI device structures and thus can easily cause shorting and other electrical performance problems.
Silver selenide is a desirable chalcogenide glass material for use in fabricating non-volatile memory devices. To date, however, there has been only limited research into the fundamental properties of silver selenide thin films. Furthermore, most of this research involves forming silver selenide films via an evaporation deposition technique. Evaporation deposition, however, is not well suited for industrial application and has an attendant problem in that the dissociative properties of silver selenide make it difficult to achieve precision stoichiometries in the deposited film.
Physical vapor deposition, also known as “sputtering,” is more easily adaptable to industrial applications and also provides better coating thickness and quality control than evaporation deposition techniques. Silver selenide, however, exhibits defect formation during conventional sputter deposition due to localized high temperatures, which occur during the sputtering operation.
Sputtering devices have long been used by the semiconductor processing industry to coat substrates (e.g., silicon wafers) with various materials (e.g., aluminum, titanium, gold, etc.) during the manufacture of integrated circuits. Generally, in a sputtering device, the material to be deposited or sputtered onto the substrate is contained in a target. The substrate is placed on a substrate support table in a sputtering chamber. Air in the sputtering chamber is evacuated and replaced with an inert gas such as argon, usually at a low pressure. An electric field is then established between an anode, such as the walls that line the sputtering chamber, and the target, which acts as an cathode (electron source). The resulting potential gradient causes electrons to be emitted from the target surface. As these electrons are drawn toward the anode by the electric field, they strike and ionize some of the inert gas molecules. These positively charged inert gas ions are then drawn toward and collide with the negatively charged target. The ions impact the target with sufficient energy to dislodge, or sputter, particles of target material into the sputtering chamber. The substrate to be coated, which is usually positioned in the chamber with its surface facing the target, receives some of the sputtered target particles, which adhere to and coat the substrate. The cloud of free electrons, inert gas atoms, inert gas ions, and sputtered target particles that exists near the target sputtering surface is termed a “plasma discharge.”
The location of the plasma discharge may be controlled using magnetron sputtering devices, which introduce a magnetic field adjacent to the sputtering surface of the target in the sputtering chamber. The magnetic field is generated by a rotating magnetic circuit located on the side of the target opposite the sputtering surface. The magnetic field acts to trap electrons in a desired region, thus producing a region of high-density plasma. This region of high-density plasma rotates with the magnetic circuit about an axis that is perpendicular to the target sputtering surface. Thus, deep erosion of the target sputtering surface occurs in the region where the high density plasma is produced, while other portions of the target are hardly sputtered at all. This preferentially sputtered region of the target is often referred to as the racetrack portion, due to the characteristic ovoid-shaped area of the target that is eroded by the high-density plasma discharge.
It is generally known that the sputtering process generates a substantial amount of energy, which results in heating of the sputtering target. This heating is caused by the high electrical potential and current applied to the target material and by the energy delivered to the target by the bombarding ions. The heat generated during sputtering needs to be dissipated; otherwise it may damage the target and other components of the magnetron sputtering device and, in the case of silver selenide targets, may cause defect formation on the target material. In one previously known approach for cooling a sputtering target, a water-tight cooling chamber is formed on the side of the target opposite the target sputtering surface. The non-sputtering surface of the target forms one wall of the cooling chamber. The cooling chamber is filled with coolant (e.g., water), which floods the non-sputtering surface of the target and dissipates the heat generated during sputtering.
During the conventional sputter deposition of silver selenide using a magnetron sputtering device, millimeter sized defects rapidly form on the surface of the target, especially in the target racetrack area. Silver selenide target surface defect formation causes increased defect formation in and on the deposited film. This defect formation results from localized heating of the target during sputtering coupled with both the relatively low phase transition point of selenide and the low melting point of silver. It is thought that the associated heating of the Ag2Se target during sputtering promotes the diffusion of silver into the Ag2Se target, which causes natural protrusions to grow on the target. These defects are then transferred to the deposited film during sputtering, causing undesired localized stoichiometric variations in the deposited film.
Thus, in view of the foregoing, there is a desire and need for an improved method of sputtering a silver selenide target with a reduced target temperature, which in turn reduces target and deposited film defect formation.